1. Field of the Invention
The present invention relates to an induction heating cooking apparatus for determining the presence or absence of a no-load state in which container eccentricity occurs in a normal heating operation and completely escapes from a predetermined cook zone, interrupts an operation of an inverter circuit if the presence of the no-load state is determined, and thus increases stability of a circuit system.
2. Description of the Related Art
Generally, a cooking appliance (also called a cooking apparatus) includes: a main body having a control board capable of determining whether a power-supply signal is received upon receiving a command signal from a user; a cooking container seated in the main body, for including food therein; and a cooking heater installed to a lower part of the cooking container or an inner side of the main body to cook the food included in the cooking container.
An induction-heating scheme applies a current signal to a coil formed in the main body, and allows an induction current to be generated in a magnetic container due to a magnetic field generated by the current signal applied to the coil, thereby heating the container. A variety of cooking appliances, for example, a rice cooker, a pan, a cook-top, a halogen range, an HOB, and a slow cooker, etc., have been designed to use the above induction-heating scheme.
An inverter circuit for use in the above-mentioned induction-heating cooking apparatuses switches on or off a switch formed of an IGBT (Insulated Gate Bipolar Transistor), applies a high-frequency current having high power to the coil, and heats the container located on the coil.
Particularly, the induction-heating electric rice cooker has been designed to prevent the container from incurring the eccentricity from the center of the coil when the container (i.e., an inner pot) is seated in an outer case of the main body. However, other cooking apparatuses such as the cook-top may incur the eccentricity of the container spaced apart from a predetermined cook zone, such that the resonance inductance is changed according to the degree of the container eccentricity. As a result, the food may be unevenly cooked, or a circuit malfunction may occur.
In the meantime, an induction heating cooking apparatus includes: a main body; a container seated in the main body, for including food therein; and a coil installed in either a lower part of the container or an inner part of the main body so that it provides the container with heat to cook the food included in the container.
The induction heating cooking apparatus includes a key entry unit for entering a cooking command to heat/cook or warm the food included in the container; an inverter circuit connected to the key entry unit, for adjusting a current applied to the coil according to the cooking command; and a microprocessor for controlling an operation frequency of the inverter circuit.
In this case, the inverter circuit has been designed to have a unique value of the coil acting as an inductor and a unique resistance value so that it has resonance inductance equal to a frequency of an AC power-source. The inverter circuit changes the inductance according to categories of a magnetic container connected to the coil or the degree of eccentricity of the container, resulting in a complicated circuit design.
If a cooking apparatus such as a cook top shown in FIG. 1 includes a container having eccentricity and is heated, the resonance inductance is changed and is different from a predetermined inductance designed in a circuit, such that the food may be unevenly cooked, or a circuit malfunction may occur.
In other words, if a radius of a cook zone is set to a predetermined radius of D, the resonance is increased whereas resistance is reduced, in proportion to a distance D from the center of the cook zone to the center of a seated point of the heating container, a variation of output power depending upon the above-mentioned characteristics will hereinafter be described with reference to FIG. 2. In FIG. 2, an x-axis is an operation frequency depending on the variation of resonance inductance due to the container eccentricity, and a y-axis is an output power depending on the same.
As shown in FIG. 2, an inverter operation frequency is inversely proportional to the output power. The inverter circuit reduces an operation frequency from an initial operation frequency (F1) of 40 kHz for an initial stable driving operation to a normal operation frequency (F2) of 23 kHz at which normal power is generated. If a voltage (i.e., an input voltage of Vin) between both ends of a coil connected to a container is less than a reference voltage Vref, the inverter circuits stops driving. This is called a small load detection state. If a cooking load less than a reference load is detected, the small load detection state is used to block the circuit from being operated, resulting in increased circuit stability.
When normal power is generated because the initial operation frequency F1 is changed to the normal operation frequency F2, if the value of D is increased, a conventional inverter circuit reduces the inverter operation frequency in proportion to the resonance inductance, such that it performs a constant output control function.
When the degree of eccentricity is increased and the heating container is seated at a specific point completely spaced apart from a predetermined cook zone, a no-load state is established. In this case, the conventional inverter circuit reduces the inverter operation frequency to a maximum operation frequency (F3) of 20 kHz, as denoted by a bold-dotted line in FIG. 2.
In FIG. 2, G1 is indicative of a state in which the heating container is located at the center of a cook zone, G2 is indicative of a state in which the heating container incurs eccentricity by a predetermined value of D/2, G3 is indicative of a state in which the heating container incurs eccentricity by a predetermined value of D, and G4 is indicative of the ratio of the operation frequency to the output power when the heating container completely escapes from the cook zone via the D point. Therefore, as the degree of eccentricity of the heating container is increased, the bold-dotted line moves from the G1 line to the G4 line.
As shown in FIG. 3, the inverter circuit reduces the operation frequency as the container moves from an initial driving period T1 to a normal operation interval T2 according to the variation of the input voltage Vin in such a way that it performs a constant output control function capable of maintaining the output power at a predetermined level, such that it can provide the container with a constant heating source.
If the eccentricity of the container occurs at T3, an input voltage Vin applied to the circuit is reduced, an operation frequency is reduced to establish a constant output control function. The operation frequency is not reduced to the minimum operation or less frequency F3 defined by a circuit designer, such that the switching operation of the inverter is maintained and the power of P3 is generated. Therefore, there arises the power higher than the minimum power of P1 to be generated when the designer determines a no-load state.
Therefore, if the cooking container incurs high-eccentricity and escapes from a predetermined cook zone as denoted by T4 in FIG. 3, the input signal Vin applied to the circuit is still higher than the reference signal Vref, so that a microprocessor mistakes as if the container was in the cook zone without detecting the no-load state, thereby maintaining the driving of the inverter.
The signal Vfd shown in FIG. 3 is indicative of a signal applied to the microprocessor. If the Vfd signal is set to 1, the driving of the inverter is maintained. If the Vfd signal is set to zero, the inverter stops operation. If the input signal Vin is higher than the reference signal (Vref), the Vfd signal is 1.
However, as can be seen from FIG. 3, although the container completely escapes from the cook zone due to the eccentricity of the container during a normal heating period as denoted by T4, the input signal Vin is equal to or higher than the reference signal Vref, so that the Vfd signal is maintained at the value of 1. As a result, the microprocessor does not detect the no-load state, such that it continuously operates the inverter circuit.
In conclusion, the inverter circuit is continuously driven under the no-load state, so that the coil is continuously heated, resulting in deterioration of circuit stability and unnecessary power consumption.